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Driving through a flash flooded
road

A flash flood after a thunderstorm in
the Gobi, Mongolia

A flash flood is a rapid flooding of geomorphic
low-lying areas - washes, rivers and streams. It is caused by heavy
rain associated with a thunderstorm,
hurricane, or tropical storm.
Flash
floods can also occur after the collapse of an ice dam, or a human structure, such as a dam, for
example, the Johnstown
Flood of 1889. Flash floods are distinguished from
a regular flood by a timescale less than six
hours.

Causes

Flash flooding occurs when the ground becomes saturated with water
that has fallen too quickly to be absorbed. The runoff collects in
low-lying areas and rapidly flows downhill. Flash floods most often
occur in normally dry areas that have recently received
precipitation, but may be seen anywhere downstream from the source
of the precipitation - even dozens of miles from the source. In
areas on or near volcanic mountains, flash floods have also
occurred after eruptions, when glaciers have
been melted by the intense heat.

Hazards

The United States National
Weather Service gives the advice "Turn Around, Don't Drown" in
reference to flash floods; that is, it recommends that people get
out of the area of a flash flood, rather than trying to cross it.
Most people tend to underestimate the dangers of flash
floods.

Flash floods are extremely dangerous because of their sudden
nature. Being in a vehicle provides little to no protection against
being swept away; it may make people overconfident and less likely
to avoid the flash flood. More than half of the fatalities
attributed to flash floods are people swept away in vehicles when
trying to cross flooded intersections. As little as two feet of
water (60 cm) can be enough to carry away most SUV-sized vehicles. In the United States, the National
Weather Service (part of the National Oceanic
and Atmospheric Administration) reported in 2005 that, using a
national 30-year average, more people die yearly in floods (127 on
average) than by lightning (73), tornadoes (65), or hurricanes (16).

The desert southwestern U.S. is
especially dangerous for both hikers and vehicles from the sudden
onslaught of water from isolated thunderstorms. These rains fill
poorly-absorbent and often clay-like dry
riverbeds. A moving flood will usually be headed by a debris pile
that may have wood branches and/or logs. Deep slot canyons can be especially dangerous to
hikers as they may be flooded by a storm that occurs on a mesa miles away, sweeping through the canyon, making it
difficult to climb up and out of the way to avoid the flood. Valley
roads frequently cross dry river and creek beds without bridges.
From the driver's perspective, there may be clear weather, when
unexpectedly a river forms ahead of or around the vehicle in a
matter of seconds.

2009 September 26 in Metro Manila primarily Marikina city,
Taguig City, and Pasig City; and many municipalities of the
provinces of Rizal, Bulacan and Laguna taking more than a hundred
lives and leaving thousands of affected residents homeless. It also
submerged several municipalities under feet deep of water for
several weeks.

2009 October 10-13 in Northern Luzon causing major landslides
in Cordillera Mountains, and submerging 80% of the Province of
Pangasinan.